Global warming is actually reducing the amount of carbon stored in forests, warns a new study.
Scientists say it is down to the fact that – just like animals – trees tend to “live fast and die young”.
The research team, led by Cambridge University scientists, said that their findings suggest that accelerated tree growth caused by a warming climate does not necessarily translate into enhanced carbon storage.
They found that as temperatures increase, trees grow faster – but they also tend to die younger.
When the fast-growing trees die, the carbon they store is returned to the carbon cycle.
The researchers said their findings, published in the journal Nature Communications, have implications for global carbon cycle dynamics.
As the Earth’s climate continues to warm, they say tree growth will continue to accelerate, but the length of time that trees store carbon – the so-called ‘carbon residence time’ – will diminish.
During photosynthesis, trees and other plants absorb carbon dioxide from the atmosphere and use it to build new cells.
The research team explained that long-living trees, such as pines from high elevations and other conifers found across the high-northern latitude forests, can store carbon for many centuries.
Study lead author Professor Ulf Büntgen, of Cambridge University’s Department of Geography, said: “As the planet warms, it causes plants to grow faster, so the thinking is that planting more trees will lead to more carbon getting removed from the atmosphere.
“But that’s only half of the story. The other half is one that hasn’t been considered: that these fast-growing trees are holding carbon for shorter periods of time.”
Prof Büntgen uses the information contained in tree rings to study past climate conditions.
He explained that tree rings are as distinctive as fingerprints: the width, density and anatomy of each annual ring contains information about what the climate was like during that particular year.
By taking core samples from living trees and disc samples of dead trees, scientists are able to reconstruct how the Earth’s climate system behaved in the past and understand how ecosystems were, and are, responding to temperature variation.
For the current study, Prof Büntgen along with colleagues from Germany, Spain, Switzerland and Russia, sampled more than 1,100 living and dead mountain pines from the Spanish Pyrenees and 660 Siberian larch samples from the Russian Altai.
Both are high-altitude forests that have been undisturbed for thousands of years.
The researchers used the samples to reconstruct the total lifespan and juvenile growth rates of trees that were growing during both industrial and pre-industrial climate conditions.
They found that harsh, cold conditions cause tree growth to slow, but they also make trees stronger, so that they can live to a great age.
But trees growing faster during their first 25 years die much sooner than their slow-growing relatives.
The negative relationship remained “statistically significant” for samples from both living and dead trees in both regions, according to the findings.
The idea of a carbon residence time was first put forward by co-author Professor Christian Körner, of the University of Basel in Switzerland, but the study is the first time that it has been confirmed by data.
The link between growth rate and lifespan is comparable to the relationship between heart rate and lifespan seen in the animal kingdom, explained the researchers, as animals with quicker heart rates tend to grow faster but not live so long.
Prof Büntgen added: “We wanted to test the ‘live fast, die young’ hypothesis, and we’ve found that for trees in cold climates, it appears to be true.
“We’re challenging some long-held assumptions in this area, which have implications for large-scale carbon cycle dynamics.”